'Way back during my formative years (1950's), there was plenty of speculation about propulsion involving nuclear reactors. The basic scheme (on which there were many variations, some of them pretty wild) involved putting a working fluid (water was often suggested) at high pressure through a small, high temperature nuclear reactor. The result would be a jet of superhot steam (temperatures over 1400 C were considered feasible) that would make possible a rocket of increased efficiency. Even wilder, a nuclear ramjet was proposed--the air is rammed through the reactor and forms the working fluid.

Of course, we know what happened: nuclear power was oversold, and there was a reaction (social, not chemical) against it. It seems, however, that much has been learned about nuclear reactors, and that, with sufficient effort, it could be used safely, and the million-to-one advantage over chemical fuels could be put to good advantage. If built and operated correctly, it is theoretically possible to prevent the exhaust from carrying any radioactive products.

Still in the realm of fantasy at this time is nuclear fusion, which has proved devilishly difficult to harness. While I do believe that it will eventually flourish, it will be a long time before a self-contained fusion reactor small enough to fly is designed. However, the long experience with submarines shows what can be done with small fission reactors--depending on how much of it the military is willing to declassify. Any thoughts about it?

Being one who is mostly interested in non-governmental spaceflight, I can't see spaceborn nuclear reactors being a technology readily available to private sector spacefarers. But I believe if we want regular manned transportation between the planets we'll have little choice but to go nuclear.
Fusion, particularly the much safer He-3 based, could make a safe and abundant power source sometime in the distant future for that type of thing. I hope we figure it out.

In the long run, there's no question that fusion is the better energy source. While there's no question that it works, as every sunrise tells us, the challenges to building a useful reactor have proven formidable. In addition, there's the problem that both fission and fusion have been oversold, leading to the widespread public skepticism we encounter today. Remember the predicted age of "power too cheap to meter?" If you do, you may also remember what "smoking bananas" means. The advantage of fission, in the short run, is the possibility of building small reactors, that could produce a large energy output over a short period of time. But I do agree that, also in the short run, it would be hard to get all the approvals necessary for the use of a small nuclear reactor for an X-prize type project. But I still hear the beckoning lure of the million-to-one energy-mass advantage.

Nuclear Propulsion
For a manned mission to a distant planet like Mars a nuclear propulsion system or nuclear thermal system seems to be more advantageous in terms of propulsive power than a chemical system as a result of some basic differences. The two main features that lead to the advantages of a nuclear thermal rocket over a chemical one are the enormous energy available per unit mass of fission (or fusion) fuel, and that in a nuclear thermal system the energy producing medium is separate from the thrust-producing propellant.

The first difference between both systems provides nuclear thermal systems with a greater specific impulse (Isp) than chemical ones. The greater specific impulse of the nuclear rocket allows it to carry a larger payload into space, and to accomplish its missions in a reduced time span. The other advantage of a high specific impulse is that the spacecraft can attain higher transfer orbits that result in a transfer orbit that minimizes travel time to the destination.

The second fundamental difference allows nuclear systems to use propellants of low molecular weight, which increase the propulsive force per unit propellant flow. The low molecular weight of the propellant permits for the use of a greater proportion of the total weight placed in space to be composed of the actual payload and not of the propellant. Low molecular weight propellants give mission designers a degree of flexibility for mission design that is not permitted by the chemical propulsion system. With the use of a nuclear propulsion system, mission designers can design missions that are more scientifically complex in nature, because more equipment can be taken up into orbit.

A consequence of these advantages is that nuclear propulsion thus allows the planification of manned missions to distant planets such as Mars. This is not possible with the use of a chemical system, because the crew would not survive the prolonged travel time between Earth and Mars that would be necessary with a rocket propelled by conventional chemical reactions.

Although the interest in high-thrust nuclear thermal propulsion systems (NTP) has grown after President Bush's Space Exploration Initiative in 1991, research into NTP systems has been going on for approximately 50 years. Among the many research programs that have focused on this type of propulsion for the past 50 years is the Rover/NERVA program. The Rover/NERVA program, which lasted for 17 years, proved the feasibility of, and built full-scale operating versions of fission-driven rocket reactors. Also, this program developed an NTP engine system. The NTP engine developed by the Rover/NERVA program was never fully tested, because the program was canceled before flight-testing was achieved.

How easy it seems that we could put one of these on one of are space ships, in send it to mars or anyware else!

The exact specific impulse, of course, depends on the kinetic energy, and hence the temperature, to which you can bring the working fluid. I recall as a kid (in the Sputnik era) that claims of Isp = 3000 s were being touted. I haven't done the calculations myself. Water is frequently suggested, both because it can transport a lot of energy and because it's non-polluting. I understand that some nuclear reactors have used H2O under ultrahigh pressure, at temperatures over 1100 C (well above the critical point). If that could be achieved in a rocket, it would probably offset the added weight of the reactor and be a viable means of propulsion.

personally, i think the only real use for nuclear reactors in space in the near (20 years) future will be for powering deep space probes and/or possibly propulsion/power on a mars mission. it'd be nice if we could get them to work for a shuttle, but the risks for that are too high with our relatively unstable reactors and launch systems. even if you can take 5000 launches with one on average before it's destroyed, that's still not good enough to justify the pollution/destruction that would occur when it blew up. the best way i see it for nuclear propulsion to be used is on larger ships built in space. for example, if you wanted to send 20 people and all the equipment required for a moon base in a single mission, it would probably be cheaper than sending large numbers of smaller missions, but you couldn't use chemical propulsion for that because the mass would be so high that you'd need an ungodly amount of fuel, which increases the weight as well, whereas you could construct or put a nuclear engine in it, and not only would it give a far greater amount of thrust, it would also weigh less than a chemical system would.

That, of course, was the idea of "Discovery" in 2001: A Space Odyssey: a huge nuclear rocket assembled in space, with the crew shielded from the reactor not by a huge mass of lead, but by distance--a very useful design if the spacecraft never has to land.

On the reliability of reactors, I have not kept abreast of the technology, but I was under the impression that the small reactors used on submarines were considered very safe and reliable. Is that not the case?

Yes, they are, but they are too heavy to use in a rocket or spaceship. There has been considderable research into nuclear powered engines, also for airplanes (ramjets). The prohibitive factor was weight v.s. shielding. However, with today's technology it would probably be feasable to build a lightweight reactor that could be used. The engines themselves worked, but the thrust to weight ratio was too low.

One major problem remains: air- and spacecraft crash. With a chemical engine, this is less of a problem than with a nuclear drive.

Research into using nuclear engines in UAV's is being done however. It would make them capable of staying in the air for months.

Perhaps we're back to the other idea--use a land-based reactor to generate outrageous amounts of electricity, and reconsider the possibility of a Maglev launch. Building the launch track is feasible with available technology, if rather expensive. Even the biggest issue--the near-megamp currents required-- could be managed. The big advantage is that the reactor and all the generating equipment stay on the ground. If we want to build a 5 km track and accelerate at 6 g, we get a final velocity of 775 m/s. If the track elevates by 2 km, we're clear of the densest part of the lower atmosphere. This replaces the first stage, with the advantage that the first stage fuel-and the reactor--doesn't need to be accelerated with the vehicle.

While onboard nuclear power may be out of the realm of private companies, nuclear thermal rockets using energy from the giant fusion reactor we orbit aren't. They are normally called Solar Thermal Rockets.

I suggest using Ammonia, as it is easier to store than Hydrogen and has a higher Isp than water. It could possibly be used as a first stage for a rocket, as well.